The invention relates generally to improvements in servo arrangements for the drive of a recording tape in broadband signal reproducing systems, such as color video recording and reproducing systems.
Our U.S. Pat. No. 3,959,815 issued May 25, 1976 for "Arrangements for Time Base Error Correction", discloses in the last embodiment thereof a magnetic tape drive servo arrangement utilizing a plurality of feedback loops, including in particular a tachometer loop and a tape lock loop. The tachometer loop includes a phase detector, the control input of which is connected to a tachometer coupled to the shaft of the capstan drive motor and the output of which is used to control the speed of the motor, by means of a frequency compensation circuit. The tape lock loop contains a phase detector which serves in playback to lock the time base pulses (for example, horizontal synchronizing pulses taken off the video tape and subject to time base error) to a stable reference frequency. The control input of the tape lock phase detector is connected to a sync separator which separates the horizontal sync pulses from the off-tape video signal. The reference input of the tape lock phase detector is connected to a stable reference which is at the horizontal video line scan frequency, and the error signal output of the tape lock phase detector is used to regulate the output frequency of a voltage controlled oscillator (VCO). The output of the VCO is connected through a divider to the reference input of the tach loop phase detector.
In the above described embodiment of U.S. Pat. No. 3,959,815 the properties and functions of the two loops complement each other. The tachometer loop is designed for a bandwidth of approximately 200 Hz and, as a result, the relatively wide band tachometer loop is able to hold the motor speed constant in the face of both relatively high and relatively low frequency variations in the speed of the motor. The tach loop, therefore, is in the nature of a coarse adjustment means. On the other hand, the tape lock loop has a bandwidth of approximately 25 Hz so as to pass only low frequency components. As a result the tape lock loop corrects for low frequency speed variations of the motor. In effect the tape lock loop stabilizes the long-time average of the horizontal synchronizing pulses by correcting changes in pulse frequency which may result from factors other than motor speed variations, such as tape stretching, tension changes and the like. The tape lock loop accordingly acts as a fine adjusting means.
The reproducing system disclosed in the above-mentioned embodiment of our U.S. Pat. No. 3,959,815 also comprises a storage-type delay line, for example a bucket brigade, by which the video signal to the TV receiver is provided with electronic time base error correction. A time base error correcting feedback loop controls the variable delay line by way of a further voltage controlled oscillator, and this feedback loop in our above patent shares at least some of the circuitry of the tape lock loop. Both the time base error and tape lock loops in our patent are taken through the delay line, both use the same sync separator, and both also utilize the same phase detector. That is, a single phase detector furnishes an error signal voltage for both the VCO of the tape lock loop and the VCO of the time base error correcting feedback loop.
While this embodiment of our U.S. Pat. No. 3,959,815 in most respects represents a highly satisfactory arrangement, we have found the foregoing partial commonality of the time base error and tape lock loops to have the disadvantage that when the tape lock loop gain is set high the tape lock loop becomes sensitive to signal dropout and the like and when the loop gain is reduced to avoid dropout problems the remaining flutter component is undesirably high.